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357 Ruminal Biosynthesis of Aromatic Amino Acids from Arylacetic Acids

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357 Ruminal Biosynthesis of Aromatic Amino Acids from Arylacetic Acids Downloaded from 357 https://www.cambridge.org/core Ruminal biosynthesis of aromatic amino acids from arylacetic acids, glucose, shikirnic acid and phenol BY S. KRISTENSEN Organic Chemical Laboratory, Royal Veterinary and Agricultural University, Copenhagen, Denmark . IP address: (Received 19 July 1973 - Accepted 15 October 1973) 170.106.33.14 I. Ruminal metabolism of labelled phenylacetic acid, 4-hydroxyphenylacetic acid, indole- 3-acetic acid, glucose, shikimic acid,, phenol, and serine was studied in vitro by short-term incubation with special reference to incorporation rates into aromatic amino acids. 2. Earlier reports on reductive carboxylation of phenylacetic acid and indole-3-acetic , on acid in the rumen were confirmed and the formation of tyrosine from 4-hydroxyphenylacetic 02 Oct 2021 at 07:13:11 acid was demonstrated for the first time. 3. The amount of pllenylulanine synthesized from phenylacetic acid was estimated to be 2 mg/l rumen contents pcr 24 h, whereas the amount synthesized from glucose might be eight times as great, depending on diet. 4. Shikimic acid was a poor precursor of the aromatic amino acids, presumably owing to its slow entry into rumen bacteria. 5. A slow conversion of phenol into tyrosine was observed. , subject to the Cambridge Core terms of use, available at The mechanisms of biosynthesis of rumen amino acids have been reviewed by Allison (1969). Most of these conform to known pathways, but new pathways have been established for the biosynthesis of glutamate (Emmanuel & Milligan, 197z), the branched-chain amino acids (Allison & Bryant, 1963), phenylalanine, and tryptophan (see below). It has been shown that all these substances are synthesized by reduction, carboxylation, and amination of their corresponding fatty acids with one carbon less. The aromatic amino acids, which all have carbon skeletons that are essential to mammals, are generally synthesized from glucose by the shikimate pathway in bacteria as well as in plants (Bohm, 1965). In experiments with whole rumen contents, Allison (1965) found a 3.4% incorpora- tion of [ ~-~~C]phenylaceticacid into combined bacterial and protozoal protein after 2 h incubation in vitro ; phcnylalanine was the only amino acid containing appreciable https://www.cambridge.org/core/terms radioactivity. He reported a three to ten times greater incorporation in similar experi- ments with pure cultures of Bacteroides succinogenes and Ruminococcus jlavefaciens. Allison & Robinson (1967) found a 3.5 yo incorporation of indole-3-[1-~~C]aceticacid into ruminal microbial protein after z h of incubation in vitro; tryptophan was reported to contain virtually all the radioactivity of the alkaline protein hydrolysates. Of a number of pure cultures tested by them only two strains of Ruminococcus albus incorporated significant amounts of indole-3-acetic acid. By analogy one would expect 4-hydroxyphenylacetic acid to be incorporated into tyrosine. However, to the best . knowledge of the author, experiments designed to test this hypothesis have not been https://doi.org/10.1079/BJN19740044 published. Recently an entirely different pathway of tyrosine biosynthesis, involving direct condensation of phenol and serine, has been described (Enei, Matsui, Yamashita, Downloaded from 3 58 S. KRISTENSEN I974 https://www.cambridge.org/core Table I. Precursors used and their incorpomtion into rumen bacteria from a cow after various periods of incubation Specific Radioactivity radio- Incorporation into bacteria (y'u) per expt activity , __ 7 Precursor (/Ci) (mCi/mmol) 15 min 30 min 60 min 90 min [U-1*C]glucose 9.6 I0 10'0 7.9 6.9 5'2 [G-14C]sliikimic acid 2'5 1'9 0.3 0.3 0'4 0.6 . IP address: [I J4C]phenylacetic acid 2-8 45 0.5 0.9 1'5 1.9 4-Hydroxyphenyl- 2.8 I0 0'9 I .6 4'0 1'9 [I -14C]acetic acid 170.106.33.14 Indole-3-[1-~~C]aceticacid 2.2 52 0'2 0.4 0.7 1'3 Okumura & Yamada, 1972). Of 1041 strains of micro-organisms testcd, they found that one Bacillus species and twenty-one strains of gram-negative bacteria possessed , on the relevant enzyme, tyrosine phenol lyase. Incorporation of serine into tryptophan is 02 Oct 2021 at 07:13:11 obligatory in tryptophan biosynthesis by the shikimate pathway. The ability of rumen micro-organisms to perform this reaction has been demonstrated by Candlish, Devlin & LaCroix (1972). The biological significance of the above observations is uncertain. Phenylacetic acid, 4-hydroxyphenylacetic acid, and indole-3-acetic acid are major products of , subject to the Cambridge Core terms of use, available at ruminal catabolism of the aromatic amino acids (Scott, Ward & Dawson, 1964), and reductive carboxylation of these acids may be a means of salvaging breakdown pro- ducts of fodder protein. Oltjen, Slyter, Williams & Kern (1971) observed an increase in nitrogen retention in steers fed on purified urea and soya-bean diets upon addition of the sodium salts of 2-methylhutyrate, isovalcrate, isobutyrate and phenylacetate ; the salts were found to raise plasma concentrations of the branched-chain amino acids, but the concentration of phenylalanine was unaffected. The present investigation was undertaken to evaluate the relative importance of reductive carboxylation compared with the shikimate pathway in ruminal biosynthesis of aromatic amino acids. Some experiments were included to test the possible signi- ficance of the tyrosine phenol lyase reaction. https://www.cambridge.org/core/terms EXPERIMENTAL Animals and incubation Rumcn contents were obtained, by suction through a tube provided with a coarse sieve, from a fistulated heifer (Jersey, z years, 300 kg) or a cow (Red Danish, 5 years, 500 kg) and the samples were kept in a pre-warmed vacuum flask before use (maxi- mally 0-5 h). All samples were taken 3 h after the morning feed. The heifer was given 8 kg hay (experiments with glucose and shikimic acid) and the cow was given 3 kg oats and 8 kg lucerne pellets with free access to straw (experiments with arylacetic . https://doi.org/10.1079/BJN19740044 acids). All main types of rumen bacteria should be present with either diet, though the proportion of different types might vary (J. L. Wolstrup, personal communication). Rumen contents (5 ml) were incubated at 39O under a constant stream of CO, which was deoxygenated by passing it over copper shavings heated at 300'. Precursors were Downloaded from Vol. 31 Ruminal biosynthesis of amino acids 359 https://www.cambridge.org/core INACTIVATED SAMPLE OF RUMEN CONTENTS ;SUPERNATANT FRACTION PROTOZOA AND PLANT DEBRIS + + Bacterial pellet Cell-free fluid . IP address: LOW-MOLECULAR-WEIG HT METABOLITES $--+ DIETHYL ETHER EXTRACT AQUEOUS (containing RESIDUE 170.106.33.14 LIPIDS aromatic acids) Ion-exchange NUCLEIC ACIDS + , on AMMONIA ELUATE WATER ELUATE 9 02 Oct 2021 at 07:13:11 PROTEIN Paper electrophoresis and chromatography to Hydrolysis4 isolate free aromatic amino acids j. Col u rn n c hromatograp h y , subject to the Cambridge Core terms of use, available at PHENYLALANINE TYROSINE TRYPTOPHAN Fig. I. Schematic- representation of the fractionation procedure. For details see p. 359--360. added in 1-0ml sterile solution containing (g/l) : RH,PO,, 3 ; K,HPO,, 3 ; (NH,),SO,, 6; NaCI, 6 and MgSO,, 0.6. The incubation was stopped after 15, 30, 60 and 90 min with 1.0ml 5 M-NaOH. Blanks were included. Radioactive chemicals were obtained from The Radiochemical Centre, Amersham, Bucks., except shikirnic acid which was obtained from NEN Chemicals, Dreieichen- hain, West Germany, and 4-hydroxyphenyl-[1-l~C]aceticacid which was prepared enzymatically from m-tyrosine labelled in the side-chain at position 2 with 14C (unpublished work). The amounts used in each experiment are given in Table I. https://www.cambridge.org/core/terms Fyactionation scheme The procedure used is summarized in Fig. I. Protozoa and plant debris were removed from the inactivated sample by centrifu- gation at 30 g for 5 min. The pellet was freeze-dried and solubilized with 1-0 ml ProtosoF (NEN Chemicals, Dreieichenhain, West Germany) before counting. The supernatant fraction was separated into bacteria and cell-free fluid by ccntrifu- gation at 18000 g for 15 min. https://doi.org/10.1079/BJN19740044 The bacterial pellet was washed three times with water (2.0 ml) and then fraction- ated by the method of Roberts, Comie, Abelson, Bolton & Britten (1955): (a) low- molecular-weight metabolites were extracted with trichloroacetic acid (50 g/1; 5 ml) for 30 min at 5"; (B) lipids were extracted from the residue with ethanol (75 % v/v; Downloaded from 360 S. KRISTENSEN I974 https://www.cambridge.org/core 5 ml) for 30 min at 45O, followcd by diethyl ether (2.5 ml) and ethanol (75 % v/v; 2.5 ml) for 15 min at 45" and the extracts were combined; (c) nucleic acids were extracted from the residue with trichloroacetic acid (50 g/l; 5 ml) for 30 min on a boiling water-bath. The residue constituted the bacterial protein. The bacterial protein was hydrolysed for 24 h at 120" in closed tubes containing Ba(OH),.SH,O (65 mg) and water (1.0ml). After cooling, the pH was adjusted to 7 with dilute sulphuric acid; the precipitated BaSO, was removed by centrifugation and . IP address: washed three times with water (2.0 ml). The supernatant fraction and washings were pooled, evaporated to dryness and redissolved in 300-500 p10.1 M-HCl. Phenylalanine (10mg), tyrosine (4 mg) and tryptophan (4 mg) were added as carriers and the aro- 170.106.33.14 matic amino acids were isolated on a Sephadex G-10 column by the method of Kowalska (1969). The column (IS x 850 mm) was eluted with 0-5 M-NaCl at 24 ml/h, and 4ml fractions were collected. The absorbance of the eluate was measured at , on 256 nm (Uvicord, LKB Instruments, Stockholm, Sweden) and all fractions were 02 Oct 2021 at 07:13:11 spot-tested with ninhydrin.
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